It is known that a laser characteristic is improved such that a quantum well active layer is strained to transform a valence band structure. However, when strain the amount of which is larger than a critical strain amount is introduced, dislocation occurs by relaxation of the strain to deteriorate the reliability of a semiconductor laser. For this reason, a strain-compensation type quantum well structure which introduces strain having a direction opposing that of a well layer to a barrier layer to prevent an average strain amount from exceeding a critical strain amount is proposed. It is reported in Non-patent Document 1, Non-patent Document 2, or Non-patent Document 3 that, in the strain-compensation type quantum well structure, an optical characteristic of a quantum well and a characteristic of a semiconductor layer are preferable at a position where an average strain amount is almost zero.
An average strain amount ε (average) is defined by the following equation:
                                          ɛ            ⁡                          (              average              )                                =                                                    ∑                                  i                  =                  1                                n                            ⁢                              (                                  ɛ                  ⁢                                                                          ⁢                                      i                    ⨯                    di                                                  )                                      d                          ⁢                                  ⁢                  d          =                                    ∑                              i                =                1                            n                        ⁢            di                                              [                  Equation          ⁢                                          ⁢          1                ]            
The number of strained semiconductor layers is set at j, and the number of unstrained semiconductor layers sandwiched by the strained semiconductor layers is set at k. In a double hetero mesa-stripe or a recombination layer obtained by stacking n (n=j+k) semiconductor layers, a strain amount of an i-th semiconductor layer is represented by ε i, and the thickness of the i-th semiconductor layer is represented by di.
On the other hand, a semiconductor laser (ASM-LD: All Selective MOVPE grown Laser Diode) obtained by all selective MOVPE (Metal-Organic Vapor Phase Epitaxy) growth is characterized in that a buried heterostructure (BH) can be manufactured without an etching process.
FIG. 8 shows a structural diagram of an ASM-LD described in Non-patent Document 4. A double hetero mesa-stripe 6 (to be referred to as a DH mesa-stripe hereinafter) including a strained multiple quantum well active layer 3a is formed on an n-type InP substrate 1 having the (001) plane as a growing surface, and the double hetero mesa-stripe 6 is buried with a p-type InP current blocking layer 7 and an n-type InP current blocking layer 8. On these layers, a p-type InP cladding layer 9 and a p-type InGaAs contact layer 10 are formed. In order to reduce a parasitic capacitance, at predetermined positions on both the sides of the DH mesa-stripe 6, two grooves reaching the n-type InP substrate 1 are formed to separate a ridge portion region including the DH mesa-stripe 6 from both the sides thereof. An n-type electrode 11 is formed on the lower surface of the resultant structure, and a p-type electrode 12 connected through an opening formed in an insulating film is formed on the upper surface of the resultant structure.
Manufacturing steps are shown in FIG. 9. Two stripe-shaped silicon oxide masks 13 (mask width: 5 μm) are formed along the [110] direction on the n-type InP substrate 1 having the (001) plane as a growing surface. In this case, since the two silicon oxide masks 13 serve as growth-blocking masks in selective growth, a narrow portion 14 sandwiched between the two silicon oxide masks 13 and broad portions 15 on both the outsides of the two silicon oxide masks 13 serve as regions which can be selectively grown (FIG. 9(a)).
In the narrow portion 14, the DH mesa-stripe 6 serving as a first semiconductor layered product constituted by an n-type InP buffer layer 18, a strained multiple quantum well active layer 3a, and a p-type InP cap layer 5 is manufactured. In the selective MOVPE growth, at the same time, a recombination layer 16 serving as a second semiconductor laminated product is formed in the broad portion 15 (FIG. 9(b)).
A new silicon oxide mask 17 is formed on only the top of the DH mesa-stripe 6 by a self-alignment process (FIG. 9(c)), and the p-type InP current blocking layer 7 and the n-type InP current blocking layer 8 are selectively grown using the silicon oxide mask 17. After the silicon oxide film 17 is removed, the p-type InP cladding layer 9 and the p-type InGaAs contact layer 10 are grown (FIG. 9(e)). Thereafter, two grooves reaching the n-type InP substrate are formed, and an n-type electrode on the lower surface and a p-type electrode on the upper surface are formed, so that a semiconductor laser is completed.    Non-patent Document 1: International Conference on Indium Phosphide and Related Materials, Technical Summary, p. 47 to 50, MoB1-2, May 16, 1999    Non-patent Document 2: Electronics Letters Vol. 27, No. 14, pp. 1268 to 1270, 1991    Non-patent Document 3: Applied Physics Letters Vol. 58, pp. 1952 to 1954, 1991    Non-patent Document 4: Electronic Materials, pp. 32 to 36, November, 1999    Non-patent Document 5: Journal of Electronic Materials Vol. 25, No. 3, pp. 401 to 406, 1996    Non-patent Document 6: IEEE Journal of Quantum Electronics Vol. 35, pp. 771 to 782, 1999    Non-patent Document 7: Journal of Crystal Growth Vol. 27, pp. 118 to 125, 1974